how hard would it be..
 

I am thinking of changing my bike so that it can use cassettes for the gears, and I wonder, is it as simple as buying a new wheel with a 7 speed hub and putting on a 7 speed cassette, and a minor adjustment to the derailleur? (which would leave me with a spare wheel)

Is the deraileur/shifters even going to know there is a difference?

yeah, probably adjust the brakes too, but that is easy.

Are you asking about going from 7-speed freewheel to 7-speed cassette? Well, that would play out about like that - assuming you stay within the same tooth counts. Although you'll probably get an 8/9/10-speed body, so you'll need a spacer or two for the 7-speed cassette to settle properly. Also, don't expect the positions of the sprockets to line up exactly. Spacing is the same, but going from one wheel to the other probably require some tuning.